1. Field of the Invention
This invention relates generally to displacement sensors and particularly to a displacement sensor for acquiring the position of two parts which can be moved relative to one another wherein the first part has at least one sensor and the second part has a sequence of spaced-apart markings that trigger the sensor.
2. Related Art
Displacement sensors are required wherever it is necessary to acquire the exact position of two parts that can be moved relative to one another. In this case, it is usual for a sensor or a plurality of sensors to be placed on the first of the two parts. A sequence of markings is placed on the second of the two parts on a scale at constant distances, and the sequence of markings can be detected by the sensors on the first part. The markings can be detected optically, electrically-capacitively, magnetically or by any other detection method. In the event of relative displacement of the two parts, the sensors count the markings that sweep past the sensors as the two parts move with respect to one another. The result is an incremental displacement sensor that is unable to determine the absolute position of the two parts. To determine the absolute position of the two parts, additional reference markings are required that can be detected as absolute reference points from which the displacement markings can then be counted in a relative manner.
For example, it is known to provide a fine thread on a hydraulic piston rod made of magnetic material with the fine thread being filled with a non-magnetic material to smooth the piston rod. The turns of the fine thread form the individual markings on the piston rod. The piston rod is guided in an annular bearing having a plurality of magnetic sensors arranged on the circumference of the annular bearing. These magnetic sensors can be, for example, of magneto-resistive design or Hall sensors. When the piston rod is displaced, the magnetic sensors react with a certain phase shift as each individual marking passes a magnetic sensor. In this way, the magnetic sensors acquire the number of markings that have moved past each magnetic sensor as the piston rod moves, and the displacement distance is calculated as the product of the number of activations of the magnetic sensor times the thread pitch.
The above described arrangement enables the resolution of the incremental displacement sensor to be increased by increasing the number of magnetic sensors in the system. The resolution corresponds to the pitch of the thread divided by the number of magnetic sensors on the circumference of the annular bearing. It is also possible to draw a conclusion about the direction of movement from the order in which the sensors respond. However, the absolute position of the two parts relative to one another cannot be calculated. To measure the absolute position, markings of a different material than the material filling the threads that can be detected as reference position are introduced in the thread. However, each additional reference marking signifies a considerable increased outlay in the production and evaluation of the displacement sensor.
In the prior displacement sensors, after movement of the piston rod occurs when the displacement sensor is in the power-off state, the absolute position of the piston rod is initially unknown after the displacement sensor has been switched on again. When displacement sensors are integrated, for example, in shock absorbers or struts of motor vehicles, problems may arise when the vehicle has been loaded with the voltage supply of the displacement sensor switched off because, when the vehicle is started, a completely incorrect position is assumed by the displacement sensor.
An example of the type of problem that may result is in vehicles that adjust the angle of the headlights based on the values received from the displacement sensor. Because the displacement sensor is reading the incorrect absolute value, the headlights are adjusted according to the incorrectly assumed absolute value and may shine at such an angle as to impair the vision of other drivers. Even in prior art displacement sensors that include reference markings to determine the absolute position, when the vehicle is traveling slowly on a flat road there may be little chassis movement, and it is possible that initially no reference position will pass a magnetic sensor where the magnetic sensors are situated exactly between two reference markings. The incorrect headlight setting may therefore be maintained as the journey progresses even when the displacement sensor includes reference markings.
An object of the present invention is to provide a displacement sensor that can, without special reference markings, determine the absolute position of two parts that can be moved relative to one another.